A thought experiment on dovetail strength
The subject of the strength of dovetail joints came up in the Woodnet hand tool forum recently, and I realized that although I had always thought of a dovetail as being a very strong joint, I never had tried to work through why that might be the case.
My assumption had been that a dovetail was strong because of mechanical strength. The dovetail joint does provide some long grain-to-long grain glue surfaces, but I had always thought that this was a relatively minor factor, with the mechanical interlocking nature of the joint being the real hero in this story.
So let’s make a dovetail joint with two 3/4” thick boards. Here’s one of them, seen from the end.
And let’s lay out some dovetails. These are going to be utilitarian dovetails, such as you might do for a crate. No thought is going to be put into trying to emulate fancy London pattern dovetails, with their skinny pins. Just plain old dovetails, laid out fairly evenly.
The darker brown lines are the surfaces that you would paint with glue, and provide the long grain-to-long grain glue bond. Remember that these surfaces are 3/4” deep, since we are dovetailing together two boards, 3/4” thick. Thanks to the miracle of Pixelmator, I can rotate these lines and lay them end-to-end.
This surprised me. As it turns out, the long grain-to-long grain surfaces add up to an area that’s larger than the area of the end of the board itself. Most woodworkers, when they think of strong glue joints, think of a long grain edge-to-edge joint such as you would use to glue up a panel. This shows that a dovetail joint can provide more glue surface than an edge-to-edge joint over the same area.
So what if you happen to be a fan of fancy London pattern dovetail joints? We can redo the layout like this.
And layout the glue surfaces like in the first example.
And we find that the glue surfaces provided by this dovetail joint work out to be just about the same surface area as the end of the board.
Of course, if you have a wide enough board and wide enough tails, you’ll get to a point where the glue surface is going to be significantly smaller than the area covered by the end of the board. After playing around with some layouts, I found that you had to get to a point where the width of the tails were about three times the thickness of the board before this would be a factor. And at that point, the joint itself just starts looking weird to my eyes. I don’t think I’m alone in this. An image search of London pattern dovetails did not turn up many examples of this joint with very wide tails.
Hi there, what's the best site to identify Japanese chisel black smiths and the makers ? Hope to hear from you soon. Kind regards.
Japan Tool and Iida Tool have good high quality pictures of a number of chisels.
Daiku Dojo has a gallery of Japanese planes identified by maker. This gallery is for planes, so this probably won’t help you with chisels.
But beyond that, there’s not much in the way of a field guide to Japanese tool stamps. I always keep in mind that back in Japan and historically, there are a lot more blacksmiths than we see here in the U.S., so it wouldn’t be unusual to find a Japanese tool with a stamp that is not immediately recognizable. In addition, sometimes blacksmiths would stamp their tools with their particular mark, but also make a secondary line with a different stamp, and it sometimes wouldn’t be clear that the two lines of tools were made by the same blacksmith.